What does a black hole look like? What will you see?

The Universe destroys something differently. If you try to hold your breath in space, your lungs will explode; if instead you breathe every molecule of air, you lose consciousness. In some places you will freeze, having lost the last heat of your body; in others it will be so hot that the atoms of your body will turn into plasma. But of all the ways in which the universe gets rid of objects, the most fascinating is to send him into a black hole.

What is beyond the horizon of events?

According to our theory of gravity - Einstein's general theory of relativity - the properties of a black hole are determined by three things. Namely:

  • Weight, or the total amount of matter and the equivalent amount of energy (according to the formula E = mc2), which go to the formation and growth of a black hole to its current state.
  • Charge, or the total electric charge that exists in a black hole from all positively and negatively charged objects that fell into a black hole in the entire history of its life.
  • Angular momentum (moment), or spin, which is a measure of the total amount of rotational motion that a black hole has by its nature.
  • In reality, all the black holes that are physicallyexist in our Universe, must have large masses, a significant amount of angular momentum and insignificant charges. This makes the situation extremely complicated.

    When we usually present a black hole, weImagine a simple version of it, which is described only by its mass. It has an event horizon surrounding a single point, and an area surrounding this point, beyond which the light cannot go out. This area is completely spherical and has a border separating areas from which light can escape and which cannot: the event horizon. The event horizon is located at a certain distance (Schwarzschild radius) from the singularity in all directions simultaneously.

    This is a simplified version of a realistic black hole, but it’s a great place to start thinking about physics in two different places: behind the horizon of events and inside the horizon of events.

    Beyond the event horizon, gravity leadsyourself as you normally expect. The space is bent in the presence of mass, which causes every object in the Universe to experience acceleration in the direction of the central singularity. If you were at a great distance from the black hole at rest and allowed the object to fall into it, what would you see?

    Assuming you managed to savethe stillness, you will see how the falling object slowly accelerates from you to this black hole. It will accelerate to the horizon of events, after which something strange will happen. You will find that it slows down, fades and becomes redder. But he will not disappear completely. He will only come closer to this: it will become dim, red and more difficult to detect. You can always see it if you look closely enough.

    Now let's imagine the same scenario, but this time imagine that you are the very object falling into a black hole. The experience of what is happening will be completely different.

    Event horizon will get bigger muchfaster than you expected, as the curvature of space will become stronger. Around the event horizon, the space is so curved that you will see a multitude of images of the universe, which is located outside, as if it was reflected and turned over.

    And as soon as you cross the event horizon, younot only can you still see the outer universe, but also part of the universe within the event horizon. In the last moments, the space will look completely flat.

    What is in a black hole?

    The physics of all this is complicated, but the calculations are prettyAndrew Hamilton of the University of Colorado, in a series of works from the late 2000s - early 2010s, are simple and elegant. Hamilton also created a series of impressive visualizations of what you see, falling into a black hole based on these calculations.

    Having studied these results, we can extract a numberconclusions, many of which are illogical. To try to comprehend them, you need to change the way the space is represented. Usually we think of him as a fixed tissue and we believe that the observer is somewhere "going down". But inside the event horizon you are always on the move. The space moves - like a treadmill - continuously, moving everything in itself to the singularity.

    And it moves everything so fast that even if youaccelerate straight from the singularity with infinite power, you will still fall to the center. Objects that are outside the event horizon will still send you light from all directions, but you can only see a part of the objects from the event horizon.

    The line that defines the boundary between whatAny observer can see it, mathematically described by a cardioid, where the component with the largest radius touches the event horizon, and the component of the smallest radius is in a singularity. This means that the singularity, even being a point, does not necessarily connect everything that goes into it, with everything else. If you and I fall into the event horizon from different sides at the same time, we will never see the light of each other after the event horizon crosses.

    The reason for this is constantly movingthe fabric of the universe itself. Inside the event horizon, space moves faster than light, so nothing can escape from a black hole. That is why, having got into a black hole, you start to see strange things like several images of the same object.

    One can understand this by asking the question: where is the singularity?

    From the inside of the black hole event horizon, in whichdirection you would move, you end up with the singularity itself. Therefore, oddly enough, the singularity appears in all directions. If your feet point in the direction of acceleration, you will see them in front of you, but also above you. All this is easily calculated, though extremely illogical. And this is only for a simplified case: a non-rotating black hole.

    And now let's get to the physically interesting.occasion: when a black hole rotates. Black holes owe their origin to systems of matter — like stars — that always rotate at some level. In our Universe (and in the general theory of relativity), the angular momentum is the absolute concluded value for any closed system; There is no way to get rid of it. When a set of matter collapses to a radius that is smaller than the radius of the event horizon, the angular momentum is enclosed within it, as is the mass.

    The solution we have here will be muchmore difficult. Einstein presented the general theory of relativity in 1915, and Karl Schwarzschild received a solution on a non-rotating black hole a couple of months later, in early 1916. But the next step in modeling this problem in a more realistic way - when the black hole has angular moments, and not just mass - was taken only in 1963, when Roy Kerr found the exact solution in 1963.

    There are several fundamental and important differences between the more naive and simple Schwarzschild solution and the more realistic and complex Kerr solution. Among them:

    • Instead of a single decision on where the event horizon is located, a rotating black hole has two mathematical solutions: the internal and external event horizon.
    • Beyond even the outer horizon of eventsthere is a place known as the ergosphere, in which space itself moves at a speed of rotation equal to the speed of light, and particles in it experience tremendous accelerations.
    • There is a maximum allowable ratioangular momentum to mass; if the impulse is too strong, the black hole will radiate this energy (by means of gravitational radiation) until it falls to the limit.
    • And the most interesting thing: the singularity in the center of a black hole is no longer a point, but a one-dimensional ring, whose radius is determined by the mass and angular momentum of the black hole.

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    Given all this, what happens when youfall into a black hole? Yes, the same thing will happen if you fall into a non-rotating black hole, except that the whole space does not behave as if it falls in the direction of the central singularity. Instead, the space also behaves as if it moves along the direction of rotation, like a spinning funnel. The greater the ratio of angular momentum to mass, the faster it rotates.

    That means, if you see something falling inblack hole, you will see that it becomes more dull and red, but also smeared into a ring or disk in the direction of rotation. If you fall into a black hole, you will be twisted like on a carousel, which attracts you to the center. And when you reach the singularity, it will be a ring; different parts of your body will meet with a singularity - on the inner ergosurface of the black Kerr hole - in different spatial coordinates. You will gradually stop seeing other parts of your own body.

    The most important thing you need to understand from thisall is that the fabric of space itself is in motion, and the event horizon is defined as a place where even if you move at the speed of light, whatever direction you choose, you will inevitably encounter a singularity.

    Andrew Hamilton's visualizations are the best andthe most accurate models of what happens when you fall into a black hole, and so illogical that they need to be looked at again and again until you begin to understand something (you really do not start). It's creepy and beautiful, and if you are adventurous enough to ever fly into a black hole and cross the event horizon, this will be the last thing you have ever seen.

    Will try? Tell us in our chat in Telegram.